text:
Is it possible to have two different makes of engine on a single twin-engine aircraft?
Is it possible to have to have two different makes of engines which are generating almost similar thrust for a twin-engine aircraft?

On one aircraft simultaneously? I read from some type certificate that it is prohibited. Maybe on other type certificate it may be allowed. Maybe you can start there.

Probably not what you're looking for, but certain piston twin models feature so-called "left-handed" engines, which are really essentially mirror images of the regular models, with the crankshaft spinning in the opposite direction.

Why would you want that?

just in case, if a company stops production of a certain engine or got bankrupt and there is an essential need to replace one out of two engines of an aircraft, is there any possibility to have some new different engine rather than replacing both and doing all other changes..  I am not sure if this really possible though..

Very similar to this question: Are all the engines on a multi-engine plane the same?

Yes it is possible.
For example :

A Boeing 777-200ER using GE engines . The first engine = GE90-94B and the second engine = GE90-115B .

This is called engine intermixing.

On 737 sometimes The more powerful engine is derated to the same level as the other engine, and we had to do full power takeoffs everytime. As long as the FMC is aware of the intermix, pilots are allowed to use autothrottle.

Here are the sources for your further reading :
Source 1
Source 2
Following is another example:

Models with engine intermix already certified

Aircraft models / Engines        Engines allowed

A310-221 / PW JT9D-7R4D1         1 or 2 JT9D-7R4E1
(48000 lb *)                     50000 lb*
                                 (A310-222)

A310-222 / PW JT9D-7R4E1         1 or 2 JT9D-7R4D1
(50000 lb *)                     48000 lb *
                                 (A310-221)

A310-308 / GE CF6-80C2A8         1 or 2 CF6-80C2A2
(57860 lb*)                      52460 lb*
                                 (A310-304)

A300B4-605R / GE CF6-80C2A5      1 or 2 CF6-80C2A3
(60200 lb*)                      58950 lb*
                                 (A300B4-603)

Note (*): Static thrust limit at sea level for takeoff with OAT 30°C.

For these aircraft models, the new engine(s) is (are) installed with no hardware change nor rating change.

thanks but is it possible that we have one GE make and another Trent/PW make with almost same thrust ??  I think it may not be advisable...

Yes, it is allowed. Have a look at Source 2, find "Models with engine intermix already certified"

Editted the answer for better understanding


text:
Did any Boeing 787 operator achieve the 20% fuel efficiency improvement over the 767?
Is there any airline which are able to get the complete 20% fuel efficiency on their 787 with respect to 767 it is going to replace, as told by Boeing.
I have read that one of the early customer like Air India are not able to get even 15% of fuel efficiency on their Dreamliners.

What does "20% efficiency" even mean for an jet engine? Efficiency is defined as useful energy out divided by total energy in, but jet engines are always rated by thrust. You can get some kind of meaningful "useful energy out" if you multiply that by the forward speed but that would be less than conventional, and anyway airlines would want to exceed the specified velocity for  peak efficiency in order to better amortize their capital and fixed costs. Do you mean 20% reduction with respect to a previous benchmark as the existing answer has assumed?

20% better than what? A 767? A 707? An A350? A DC-3? The Space Shuttle?

Yeah, its comparing with 767 and taken this as a benchmark.

It depends on the role they are using the aircraft for. Short haul it will use more fuel on more climbs out of airports etc, long haul it will be sat at 40,000ft cruiseing along. Quite like a car, it takes more fuel to get a car to 70mph (like taking off) than it does to keep it at 70mph. Using them on short haul will use more fuel. thats why the likes of AI cannot get them to work as they want - plus AI's maintenance is a little concerning for the lay person like myself, for instance VT-ANI was "undergoing short maintenance" (like a A check) for around 9-10 months, usual time for an A-check i

@LevelRiverSt, where does the question mention a jet engine? It does not. It asks for ‘efficiency’ of the aircraft. Saying ‘consumption’ would be formally more correct, but fuel efficiency is common term.

@JanHudec read the original text of the question before it was edited. Note response from OP at 3rd comment, at which point he clarified what he meant in the title. I still think the text is a little sloppy. Once you've done that, you can ask a moderator to delete all comments if you wish.

You have not defined your benchmark/reference value, but a three-year-old article suggests this was possible depending on definition even at a very early stage for some airlines:

ANA earlier this month was the first to reveal that its aircraft were producing around 21% lower fuel burn on international flights compared to the 767-300ER that the 787 is designed to replace.

They also say however:

After its first six months of service, ANA says the 787 efficiency levels are slightly up on the 20% savings originally expected at the beginning of the program. On domestic routes, the saving is 15-20%, which meets expectations, say ANA officials.

The article isn't clear on how they measure efficiency -- are they comparing fuel burn of a 767-300ER to the 787 on the same route, or are they looking at per-passenger fuel costs? Wikipedia says the 767-300ER has a typical seating capacity of 290 passengers (up to 350 in a single-class configuration), while the 787-8 seats 242 in a typical configuration (up to 375 for a single-class). So it would be interesting to know if the efficiency gains takes into account differing passenger counts between the two planes.

GIven that this is a marketing term, it's extremely likely that the measurement here is actually "fuel burn per seat-mile".  This is the metric that airlines generally use to measure and assess fuel efficiency for the airplane.  It can be a little deceptive, because you could increase "efficiency" 5% just by adding 5% more seats... but that's how these things are normally discussed.

Wouldn't the extra weight of those seats (and passengers...) cause an increase in fuel consumption, though? So by adding 5% more seats (assuming all other relevant regulations remain met) you might increase the number of seat-miles by a little less than 5%; say, perhaps, 3%.

Wikipedia's B787 article states that:

According to data from launch customer All Nippon Airways, the 787
  surpassed the promised 20% fuel burn reduction compared to that of the
  Boeing 767.

This newer (8 months old) article states that:
Air Canada and Japan Airlines are on the way to achieve it.

A Real-World example:
On a typical 3,551 NM airways distance flight a 365,000 lbs 767-300ER will take 08:40 and 92,300 lbs of fuel to get there with 216 pax and baggage (no cargo). The flight took longer due to 51 kts of head wind that turned the 3,551 NM into a 3,973 NM air miles (ESAD) flight.
A 424,000 lbs 787-8 will do the same mission in 07:56 and use 86,500 lbs of fuel while transporting 237 pax and baggage (no cargo). And because it flies faster, it spends less time in the 51 kts head wind and only covers 3,875 air miles (ESAD).
The 767-300ER burned 10,600 lbs per flight hour, whereas the 787-8 burned 10,900 lbs per flight hour. More! Except it does so covering more distance as it flies 6% faster.
With (3,973 x 216) seat-miles, the 767-300ER burned 0.1068 lb/seat-mile.
The 787-8 burned 0.0942 lb/seat-mile based on (3,875 x 237) seat-miles.
The 787-8 is thus 11.8% more efficient per seat-mile.
When dropping the pax load of the 787-8 to the same 216 as in the 767-300ER, we will get a tube-for-tube comparison; 21 pax weigh 4,851 lbs and lowering the payload on the 787-8 by 4,851 on an 8-hour flight will lower trip burn by approx. 1,160 lbs. The burn per seat-mile now changed to 0.1019 and this is only 4.6% more efficient than the 767-300ER.
So... marketing hype.
The 20% efficiency gain was based on fuel burn per seat-mile, not tube-for-tube numbers.
In this example it would have taken 20 more pax in the 787-8 to confirm the 20% claim.
In other words, have the 787-8 fly with (237+20) 257 pax. The higher fuel burn of 87,700 lbs (due to 20 more pax on board) would be offset by the huge increase in seat-miles and in that way reach the marketed 20% difference in efficiency.
Hope this helps.

+1, but the answer would be even better if you revealed the sources for your numbers.

Lido real-world flight plans, ran by myself.

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What is the role of baffles in Fuel Tanks?
I think there are many baffles inside the shape of Fuel tank. What kind of advantage they do?  Do they help in managing the oscillations in-flight?

Also has detail on baffles - http://aviation.stackexchange.com/questions/1183/do-fighter-jets-have-a-traditional-inverted-fuel-system

They help in keeping the fuel from sloshing around during maneuvering. Some sloshing is impossible to avoid, but a reduction is extremely helpful in avoiding unwanted dynamics.
Consider a sudden pitch-up into a climb, or a sudden acceleration when the afterburners are lit: Now the fuel will rush backwards, shifting the center of gravity backwards. If this happens suddenly and the fuel starts oscillating, the aircraft would fly a wavy flightpath without pilot input. Now add the input of the pilot who tries to keep the flightpath steady, which is delayed by maybe 0.2 seconds, and add the latency of the control system, and you might have ideal conditions for pilot induced oscillations.
Some combat aircraft don't use baffles, but a coarse, sponge-like foam which takes up maybe 2% of the tank volume but dampens fuel oscillations very effectively.

+1 for that last part. Where can I read more about this sponge foam?

@DrZ214: I only know it is from a Belgian company and used in the MRCA Tornado. Any open-cell foam which is not dissolved by aviation fuel should work.

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Which compressed air and fluid are generally used by oleos?
Which compressed air and fluid are generally being used by oleo-struts?

Generally hydraulic oil and nitrogen (to reduce corrosion of the internal cylinders and seals).

I'm sure it varies by aircraft type, but for the C172S nose gear oleo (for example) it's air and hydraulic fluid. The operating handbook says:

Keep strut filled with MIL-H-5606 hydraulic fluid per filling
  instructions placard, and with no load on the strut, inflate with air
  to 45.0 PSI

Generally Boeing struts use military hydraulic fluid (red oil) MIL-H-5606 and dry nitrogen as the charge. Many military aircraft have upgraded to synthetic hydraulic fluids that are fully compatible with MIL-H-5606, like MIL-PRF-83282 or MIL-PRF-87257. Sooner or later this will probably get adopted by airlines as well.


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What kind of heaters are used for heated floor panels?
In some Airbus planes, they have heated floor panels. So, what kind of heaters are being used there?

The warm air from the AC through a cavity would be cheapest, lightest and safest.

Do you mean the cabin is fully heated by devices in the floor, or just heaters at the doors (to prevent icing - Also used in flight decks). Can you provide a reference to an aircraft model?

Related: Patent transferred from Goodrich to Airbus.

Electrical, connected to one of the AC buses. This is an example, absorbing 620 watts/m^2.


text:
Do we have a unit to measure turbulence?
Do airplanes have any way to quantify the turbulence like if shakes are there or the some kind of air drift is happening every parameter of these have some units but do collectively there is any such unit which can quantify the turbulence?

Sick-bags per hour is a useful metric.

Shrieks per minute?

I think you'll find that both sick-bags per hour and shrieks per minute have to be adjusted by the number of passengers, to sick-bags per passenger hour and shrieks per passenger minute.

Turbulence can be quantitatively expressed as Eddy Dissipation Rate (EDR). EDR can be calculated from available onboard measurements, however it is not yet common for aircraft to have software to calculate the EDR.
Originally it was thought to have EDR reported over ADS-B, but it did not become a part of the current specifications. It might be in the next ADS-B version.
NASA Langley paper
World Meteorological Organization (WMO) presentation

Turbulence is described using the following qualitative descriptions  [Source]:

Informally, you can use a g-meter to quantify turbulence, at least in the up-down direction, but I don't know of any particular values that equate to various types of turbulence

I will try to search again as soon as I have time, but IIRC Airbus defines "severe" anything above 0.5g

What... what does the push button do? I feel compelled to push it.

resets the + and - pointers back to 1G

ah, so the top and bottom dials are min and max (since last push on the push button).

if by "top and bottom dials" you mean "top and bottom pointers" the answer is "sorta."  The top is max positive G and the bottom is max negative G. min is going to be either 0G or +1G, depending on your frame of reference.

Why no severe or extreme chop in that table?

think of chop as like driving on a gravel road. its fairly uniform and you can still drive fast on it, but it produces constant vibration.  turbulence is like potholes and speed bumps

I think you are asking two questions in a single way. I understood that you would like to know firstly is there is a physical unit that could give you quantifyable information about airplane turbulence and, on top, you are asking if, having this unit, is possible to quantify turbulence in flight.
For the first one, you already have nice answer about them. You can quantify by using EDR or some other similar units.
To the second part of the question, can I actually in flight measure it?, the answer is more complicated. Is possible to measure the turbulence once the airplane is affected by it using its effect on the airplane.
However, to the question, can I measure the turbulence before airplane flies over it? that's more complicated, and there are some R&T projects trying to investigate LASER based detection both short range and long range (30km). There is an European project called DELICAT working on this topic.


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What purpose do wing root seals serve?
What is the use of wing root seals? What kind of leaks are possible without having wing root seals, and if the seal is broken what kind of issues can airplanes face?

We just replaced ours and while talking with the A&P came up with the hypothesis that there are three reasons for it: noise reduction, small increase in speed from reduced turbulence at the root, and keeping water out of the joint.

@JScarry This is the basis for an excellent answer.

@PeterKämpf I can’t quote an authoritative source for my comment, so it’s just speculation at this point.

One thing the wing root seal does is keep water out of the join between the fuselage and the main spar. Water in there could corrode the spar or, if it is wooden, cause it to rot.

That's nasty. Does that apply elsewhere?

it's vitally important to keep water out of airplanes- so any break in the fuselage structure needs to be well-sealed.

Water will soften the epoxy matrix but otherwise will only corrode the metal parts, namely the fittings which connect the pushrods in the wing with the linkages in the fuselage. An aluminium structure will not be affected, unless you are close to the sea so that sea salt is added to the water.

Depending on the weather, don't also forget freeze-thaw erosion: small amounts of water in the tiniest of cracks and joints can cause big problems over time.

The wing root seal reduces drag.
In glider aircraft with detachable wings or foldable, the seal is often nothing more than tape.

do we have any documentation stating that it reduces the drag and can we quantify that?

@NitinG That's a good question that I'd like to know the answer to.  That sealing gaps improves performance is widely believed.  Here's one sailplane pilot documenting the sealing of gaps:
http://members.goldengate.net/~tmrent/soar/docs/135/seal135.htm .  See the last picture on the page for wing root sealing.

When building kits I have more than a few times come across the explanation of the seals being for water and element protection. It depends on the installation, on something like a Cherokee it would help reduce drag but be most effective in keeping the wing attachment dry and protected from the element, on a composite it finishes off the smoothed out junction but still allows for flexing, there are many installations where the rubber seal is used to allow for movement.

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text:
How are leaks in door rubber seals detected?
There are few reported incident where the rubber seal on the door was damaged and it took numerous hours to find the cause and then find the location of the damage. I want to know the ways to detect the leaks mostly for the big passenger jets.
I am not sure if this can be detected during maintenance checks or even aircraft is in air.

mostly jets..   edited question accordingly.

Pressurized aircraft often leak like sieves once they get older.  It's difficult to set limits unless an OEM provides a specific procedure, and an OEM procedure may be designed for new a/c as a quality control measure without any latitude for service deterioration.
There isn't really any guidance on leak rates in the FARs for cert purposes. There is only language limiting exposure time for passengers to low cabin pressures for emergency descents, which has an indirect impact on leak rates.  Beyond that it's just the ability of the bleed sources to keep the cabin pressurized.
From an operational standpoint, a leak will get looked at either when there are pressurization irregularities like a crew writing up a "slow to pressurize" snag of some kind, or when there are complaints about noise or drafts from door seals.
An airline may do a fuselage leak check on a heavy check interval like C Check (typically 5000 hrs) where they will pressurize the fuselage in the hangar (or use a vacuum system as ymb1 mentioned) and go around listening and spraying with a soap bottle.  It'll depend on their maintenance program.  If there are no service limits for leak rates however, they probably wouldn't do that kind of test if there are no pass/fail limits and will only do it as a troubleshooting exercise.

One common way to detect door seal leaks (in aircraft, cars, refrigeration, and so forth) is to (slightly) pressurize the cabin or storage space with a tracer gas and use a detector to look for locations where the gas passes the seal.  Obviously, this is easier with a smaller space an/or shorter total seal length, but if the gas is something common and cheap like carbon dioxide, it may be practical for even a full size airliner cabin.

The old way was to use a lit cigarette and watch the flow of the smoke.

@user71659 That would be a "common and cheap" tracer "gas".  For nonsmokers, incense or a punk stick would also work.


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ETOPS 330 vs ETOPS 370 etc; how are the exact minute ratings derived?
I am curious how the ETOPS minutes categories are arrived at. i.e. Say, ETOPS 330 vs 370.
What extra abilities would the 370 type have demonstrated that the 330 didn't have? Was it just that the test flight on single engine was longer? Or something else in the design limits the type to a 330 minute rating while on the drawing board? Is it the MTBF of a component? Or is the 330 minute time back calculated based on actual empirical testing and the associated failures?
On ground and maintainance procedures is there any substantial difference when dealing with a ETOPS 330 aircraft vs and ETOPS 370 aircraft?
Could there be (say) an ETOPS 430 in the future?

Some elements of the answer: At least the cargo fire suppression time and the in-flight shutdown rate (IFSD) have to be better. ETOPS time (now renamed EDTO by ICAO) isn't limited, though there is no point to go further than what is necessary to cover the south pole area between Latin America and Australia. See Module 4 of this series of ICAO documents.

Thanks @mins. I'd love to find out if anyone knows which components of say an aircraft type had to be changed for the in-flight shutdown rate when ETOPS 370 was obtained instead of ETOPS 330.

If there is a formula currently used to generate an ETOPS time, no regulatory agency has made it public that I can tell. ETOPS originally grew out of a "60-minute rule" that was the result of earlier distance-based rules (see this report for a thorough history). My guess is that 60 minutes was a case of  "well, that sounds good" rather than the result of any rigorous analysis. From there, it's mostly been manufacturers slowly pushing and getting approved for longer and longer extensions (usually just a percentage increase that results in round-ish numbers—again, this is policymaking, not engineering).
However, take a look at the formulas on pages 9–11 of this report. I hesitate to declare them the way to generate an ETOPS number because the report is thirty years old and a lot of it is critical of the methodology, so the regulators may have changed their approach between then and now. In fact, the report does not explicitly state that the formulas within were in use at the time, either. Nevertheless, if there is a formula in modern use, it likely uses a similar approach of quantifying the probabilities of failure and using a factor to convert them into a do-not-exceed diversion time.
The difference between certification at 330 minutes (777) and 370 minutes (A350) may be due to FAA vs. EASA methodologies and/or decreases in inherent failure probabilities as technology matures. With that, I doubt there is much difference in maintenance of a 777 vs. an A350 (at least as far as ETOPS goes—obviously their construction necessitates very different maintenance programs), as that 40-minute increase for the A350 was approved without any in-service maintenance data. It may be different for the 787, which was not initially certified at 330 minutes.
Going forward, I have no doubt we'll see ETOPS 430 and above. Airbus already toyed with 420 minutes for the A350, and perhaps we'll see that after some time in service. At 420 minutes, there are only a few non-compliant routes; I expect that, eventually, any new twin will be able to fly unrestricted between any two points within its endurance.

Great answer! I didn't even know there were single engine designs in scheduled operations. Which  ones can you elaborate?

@Peter Schilling. There will never be single engined aircraft carrying 10s or 100s of passengers. It got nothing to do with power and efficiency, and every thing to do with reliability. Think what happens when an engine failure occurs. While IFSDs are rare on a per plane basis, there is at least on every day. See Aviation Herald:  https://avherald.com

Not sure how I missed these comments almost a year ago, and also not sure why I went on a tangent about single-engine designs; I've removed that part of the answer.

The answer ETOPS minutes categorised arrived from the Engine failure diversion range and the time is calculated based on OEI (One ENgine Inoperative) max range distance or other speed selected by applicant.
I do agree with previous answer there is no point to go to 300+ minutes.  see image.

The north and south pole concern is more critical and additionally there is Polar certification where the operator have to prepare the polar survival suit for crew member and additionally passenger arrangement incase they divert to this remote airport in advance (such as food, clothing, accommodation, etc)


text:
Seatbelts-on during taxi protocol: Risk assessment
Typical taxi speeds are less than 20 knots. These are speeds at which city-transport  buses regularly operate & even exceed. Buses don't mandate seat belts & in fact even allow standees.
Now my question is: Is the risk of collision higher in a taxing aircraft? Are decelerations higher? Or do we operate in aviation with a lower risk appetite.
Just curious.
As a counterexample, during flight even if the seatbelt light is turned on, toilet-visits seem allowed (mostly, unless its severe turbulence). On the other hand, the ground taxi period seems an absolute PAX movement prohibition period akin to takeoff & landing.
Is ground-taxi riskier than in-flight seat-belts-on periods?
PS. Don't interpret this as advocacy to get rid the seat-belts during-taxi rule. Just curious about the differential risk assessment.
In any case most safety prohibitions are a risk-vs-convenience tradeoff. Just evaluating that tradeoff in this particular case.

There's a reason commercial air travel is the safest form of travel

@TomMcW That aligns with my "lower risk appetite" theory then. Ironic that the very buses they use to take passengers to aircraft parked at remote stands regularly exceed 20 knots & don't have seat belts & are very often stuffed full of standees. :)

The real danger would be moving luggage from overhead bin

I spent all day yesterday at an amusement park. I can't count how many times I heard, "Remain seated with your seatbelt fastened until the vehicle comes to a complete stop." I was hearing it in my sleep!

The fact that a bus don't have seat belts doesn't mean that seat belts would be useless. In my country, for a long time taxi drivers were allowed to drive without their belt on. But the risk was present. Now they need to fasten belts like any other driver. Just a matter of a corporation lobbying law makers.


text:
Would there be an efficiency advantage to weighing passengers?
In order to estimate the total weights of aircraft I know that the dispatchers use average weights for adults / children etc.
But since this is an average I suppose there's a standard deviation and sufficient allowances must be kept for safety etc. i.e. Factors of safety since the estimation itself is imprecise.
Question: I am wondering how much would be the efficiency advantage (if any) if a commercial airline decided to have weighing scales at check in for exact weight determination of every PAX? By efficiency I mean if you knew the exact passenger weight, you wouldn't have to use as large a factor of safety (conceivably) and hence could accurately pack in more cargo in the holds.
Are any airlines actually doing this? Or is the benefit insignificant to justify the modification? I mean airlines do go pretty far to ensure they can carry max payload right?
Note that I'm not asking about charging PAX by weight. Just about exploiting your load capacity to the fullest by reducing estimation errors.
If not  during check in, is there a safety advantage to incorporating a scale in the jetbridge to track total weight going in? Just to guard against dispatcher errors?
Alternatively, what about load cells in the wheel suspension itself. That would not only give the pilot the accurate weights but the relative loads would also indirectly indicate the position of the CG?  Is that impractical or too expensive?
There's already a weight on wheels sensor, correct? How difficult would it be to convert it to a quantitative sensor calibrated for newtons?
This question was partially motivated by this related SE Aviation question :
How does a commercial airliner measure its weight/mass?

MEL item : one scale may be inoperative provided another one is operational and within 12 months from last calibration.

I would think the time spent weighing passengers during boarding would far outweigh any savings from efficiency.  Not to mention that the first airlines to try this will have to deal with the inevitable objections and loss of business from the invasion of privacy.

The "weight scale on the wheels" idea was tried, somewhere in the 60s~80s IIRC. The engineers found that the scales need calibration quite frequently to provide a useful accuracy, and the idea was soon abandoned.


text:
Are helicopter searchlights fixed or movable?
Can the helicopter crew move the helicopter searchlights or are they always fixed at a specific location on the helicopter?
They are used by rescue helicopters mostly but I am not sure if crew can move them to look at specific area by some method or they are always fixed.

All light fixtures on a helicopter are fixed. Some, like search lights, can be pointed.

To make your question clearer, you might like to add a picture of a helicopter that shows the lights you're asking about.

For future reference: strobe lights are the 'flashing' lights that flash white every second or so.

The question is some what unclear, but helicopters usually have some combination of 3 types of lights.
Landing light(s); fixed, sometimes pulsing, used for illuminating the area directly in front of and below the helicopter for landing.
Anti-collision light; fixed red and green and a strobe on the tail and sometimes the belly.
Search lights; directable, usually only used on military or para-public helicopters that fly at night. [SAR, Police, etc...]

By "searchlight" I assume you mean the high intensity units used to identify ground objects at night (example: the "NightSun" series). Those are usually fully steerable with their motion often slaved to a steerable video camera. Control is done by someone other than the pilot who is sufficiently busy flying the aircraft.


text:
Are turboprops more efficient than piston engines (thrust per fuel consumption)?
I want to compare two types of engines: Turboprops and piston engines. I want to know which one is more efficient. There are many types of efficiencies, so I will specify: thrust (in newtons) per fuel consumption (kg/second).
Right now I don't care about power/weight ratio or max speed or horsepower. Those are all valid concerns for aircraft design, but I'm trying to pick exactly one criteria and figure which engine is more efficient in its terms and its terms alone. I don't want to mix criteria. I want to focus on one aspect, get an answer to it, then move on to the others later.
A complication is that piston engines usually give their output in horsepower. I have no idea why this is. What's ultimately needed is thrust. I realize that a propeller engine's job is to create torque, and thrust will depend on blade length and airspeed. In that case, I can't understand why those engines don't specify their output in torque.
Citing engines that actually exist (and citing their specs) will answer this question as long as the choices are modern, good engines.
(I did not include turbojets or turbofans for fear of being too broad. I picked two propeller engines and don't want to mix them with jet engines.)
One more thing. I realize that engines are capable of varying levels of thrust. It's my understanding that max thrust is usually more inefficient than cruising thrust. Therefore, I will ask for thrust/fuel-consumption figures of an engine at typical cruise conditions.

Turboprops (and the closely-related turboshaft) are usually also rated by power output, sometimes specified as shaft horsepower. The reason is simple: these engines are usually made and sold to accommodate a range of propellers and different props will deliver different cruise thrust levels at different speeds on different airframes.

No, piston engines are more efficient.
Their output is given in kilowatts or HP because this does not change much with speed, unlike the thrust of a propeller. By running a piston engine a few times on a dynamometer stand you can get reasonable numbers which are valid over the full operating range. If you want to characterize them by their thrust, you would need to look at an engine-propeller combination at one particular speed, which is not very helpful.
Now you ask for specs of contemporary engines. The funny thing is that the efficiency of aviation piston engines has not changed much over the last decades. If you assume 250 g of fuel per kWh at full power for gasoline engines, this figure holds already for good WW II-era engines like the Jumo 213 A which was run on 87 octane gasoline. The low octane number restricted compression to 6.93:1, while the higher octane number of contemporary AVGAS allows for an 8.5:1 compression ratio in engines like the Lycoming O-360, which consumes 280 g of fuel per kWh. Adding fuel injection enabled Lycoming to reduce fuel burn to 240 g per kWh in the IO-390, which was first run in 2002.
Diesel engines are even more efficient; they typically consume 220 g per kWh. This low value was already possible with the venerable Jumo 205 of the 1930s which consumed only 213 g per kWh at its most efficient speed. Modern aerodiesels run at similar efficiency: The Thielert range of engines which have been taken over by Continental consume 220 g per kWh.
Even the best turboprops rarely achieve less than 300 g per kWh. The most modern version of the venerable Pratt&Whitney PT6 consumes 308 g per kWh, and only very recent developments can close the gap to piston engines. The Progress D27 claims a specific fuel consumption of 231 g per kWh while the Europrop TP400 one of 237 g per kWh. Note that here the remaining thrust from the exhaust has been converted into an equivalent power rating to achieve such good values.

Okay, well assuming the engines actually use that consumption during cruise, this answers the question. Meanwhile I have to ask the related question: http://aviation.stackexchange.com/questions/29611/how-to-calculate-the-thrust-of-a-piston-or-turboprob-engine

The higher efficiency of diesel engines is illusory, and is an artifact of measuring fuel by volume instead of mass.  Diesel fuel has a higher energy volume (joules/litre) than gas, by about the proportion you quote.   This also shows up "at the pump" with diesel priced about the same proportion above gas.  See also the way copper wire has half, or double, the conductivity of aluminum wire, depending on whether you compare by mass or volume.

@Harper: Yes, Diesel fuel has a higher specific mass, but you forget the higher compression ratio of Diesels which makes them inherently more efficient. Poor individual designs are no excuse.

@Harper: also notice that the answer is specifically about g/kWh instead of liters/kWh.

Diesel engines can burn as little as 160g/kWh, like the Wärtsilä-Sulzer_RTA96-C. I've never seen one of those flying, though.

@BrianDrummond Because it weighs 2600 tons. For a more elaborate reasoning see this answer.

There is no small turbine engine that has anywhere near the specific fuel consumption (Lbs/Hp?Hr or Grams/Kw/Hr) of a gasoline or diesel aircraft engine.  Turbines are light (lbs/Hp or KG/Kw) but burn considerably more fuel.  Turbines are also extremely smooth, almost no vibration.  But as to efficiency, even our antique technology small aviation piston engines, turbines just aren't very good.
Typically the smaller the turbine the worse the specific fuel consumption.  So the Allison, now Rolls, C250 family, are terrible, sometimes getting 0.8 Lbs/Hp/Hr in low cruise setting.  The big turbines, like the AE2100, can get down to 0.4, but these engines are rated at 5,000hp, much too big for light aircraft.
One particularly troublesome aspect of turbine specific fuel consumption is that it is worse at lower power settings.  This is one of the reasons that most turbine aircraft tend to fly as high as possible, because at high altitude they are running much closer to max power to achieve best aerodynamic efficiency.  Piston engines tend to achieve their best specific fuel consumption at relatively low power settings, so flying low and slow does not hurt range nearly as much as on a turbine aircraft.
With several new diesel aircraft engines coming into use the efficiency of piston aircraft engines has gotten much better.  Some of these diesels have specific fuel consumption in the 0.35 lb/Hp/Hr range, better than even the most efficient large aircraft turbines, and close to half of a small turboprop at low cruise.

Above graph is from Torenbeek, Synthesis of Subsonic Airplane Design:

Fig. 4-1 shows the quantity of fuel used per hour by some representative examples in the categories mentioned above, the figures being for cruising flight at a given thrust which is equal to the drag.

The graph shows that turboprop engines have a slightly higher fuel consumption than piston engines, which at the time of publishing of the book were exclusively avgas engines. The graph shows the state-of-the-art of 40 years ago.

If you want to rate an engine by the thrust it makes, you must know the propeller efficiency at the given criteria for your test.  The engine performance and thrust performance both change with altitude and speed.  Only a jet engine or turbofan engine should be rated by thrust, and even then, it changes with altitude and speed.  Don't accept book numbers for prop efficiency, because it changes all over the map.

I went and broke down how much fuel is burned to create one hp in the lycoming 720 vs the pt6 and the piston engine came out to be between 7-8 percent more efficient than the turboprop.
Pt6 approximately 088-.089 gallons per HP per hour
Lycoming 720 approximately. 081-.082 gallons per HP per hour

Do you have any sources?

Adding sources of your data helps. Also, for good comparison, it should be in units weight, not units volume.


text:
What turbine blade alloy did the Jendrassik Cs-1 use?
Wikipedia says that the first turboprop aircraft engine was the Jendrassik Cs-1 invented in Hungary by György Jendrassik.
It says it was supposed to run at 1000 hp but "combustion problems" limited it to 400 hp.
I would like to know what turbine blade alloy it used. Could not find this info online. I have a feeling the blade alloy limits were what limited the engine power, since that is the main limiting factors on gas turbines in general.
Note: The Hungarian version of the wikipedia article says the turbine ran at 550 C, and...

A turbinalapátokat 50% reakciófokú csavart, szárnyprofil keresztmetszetű hőálló acélból gyártották precíziós öntéssel, és speciális beerősítéssel szerelték a rotorba.

Google translate:

The turbine blades are made of 50% reactive twisted, heat-resistant steel with wing section cross-sections, and are specially mounted in the rotor.

Don't know what that means. 50% steel? Then what is the other 50%? Also, there is no citation for it.

Turbine temperature mostly limits efficiency. Size and fuel consumption limits power.

Also, turbine has nothing to do with combustion, and the wiki page stated the room for improvements are the flame cans not turbine.

@user3528438 Yes I know that turbine temps and combustion problems are different things, but this is wikipedia and they confuse engineering things all the time. Most of it is uncited. The part you referenced, There was nothing inherently wrong with the design, however, and continued work on the flame cans should have allowed it to develop to full power. is uncited.

To the downvoter, what is wrong with the question? I don't see anything unclear or off-topic.

My money would be on Tinidur, a 30% Ni, 15% Cr alloy developed in 1936 at Krupp. Since it also contained 2% Ti, the name was a combination of Ti + Ni + dur from latin durum = hard. If György Jendrassik had asked the obvious sources of his time, that would had been the recommendation. British Nimonic 80 came 5 years later. Sorry, no source and no English Wikipedia page. Learned of the Jendrassik Cs-1 only today.

Looking up the Jendrassik CS1, Google belatedly brought me to this question; though my Hungarian is non-existent, I suspect the '50% reaction' refers to the turbine's design parameters; a 50% degree of reaction means the pressure drop across a turbine stage is shared equally between stator and rotor (i.e. between fixed and moving blades). As to materials, the turbine blading would probably have been an austenitic stainless steel of some kind; IIRC most of the early British gas turbines used 'Rex 78' stainless blading, which was an austenitic steel developed by Vickers-Firth.
Looking at the CS1's design, I very much doubt it could have been turned into a viable aircraft engine without extensive development and/or redesign. With 15 compressor stages the stage loadings would have been low, but then you'd run into all kinds of surge problems as on a single shaft many of the stages would be running away from their optimum conditions, especially as the engine didn't have a free power turbine. The combustion chamber doesn't look long enough for full flame development, and annular designs are notably tricky to get right.

Welcome to AV.SE!

What is the time frame ? Vitalium ( cobalt + chromium + Mo ) was one of the first turbine blade materials. Otherwise 50 % iron is not unusual ; for example the Incoloy group. But most of the high temperature materials have nickel as the major element . And contain Cr, Mo, Co , W , etc , and 1 to 20% iron.  A temperature of 550 C / 1022 F is certainly not very hot in the turbine world so an alloy like  Incoloy 800 H  with 45 % iron plus 34 Ni, 21 Cr,1 Al, etc  is reasonable. The relatively low temperature suggests an older design : So actually "steels" are a possibility. Stainless 310 ( 25 Cr , 20 Ni , balance iron ) and the high carbon cast version HK . And even stainless 309 ( 25 Cr, 12 Ni , balance iron ) , cast as HH. These alloys are used in great quantity as petrochemical furnace tubes at temperatures to 1700 F +. I believe I examined failed 310 blades or vanes from a mid-eastern stationary gas turbine in about 1970.

Since it seems you couldn't be bothered to read the linked Wikipedia page: The Jendrassik Cs-1 was designed in 1937 and ran first in 1940. That is 20+ years before Incoloy.

I did not look at the history. But what I meant by "Incoloy" is not a product of Inco but a large group of Fe, Ni, Cr alloys with more iron than nickel ." Tinidur" is an example , I was familiar with essentially the same alloy under the name "A 286" which I have recommended for high temperature bolting. That is why I wrote "Incoloy GROUP" intending to cover a category with many different  names which would address the "50 % steel" in the question.

A286 was a post-war US derivative of Tinidur. Also no candidate.


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What adds more weight to a twinjet? Tail-engines or wing-engines?
Let's say we have a large commercial jet, with exactly 2 engines, capable of holding at least 200 passengers.
It could be a traditional jet like the Boeing 777, where both engines are in the wing. Or it could be like the MD-90 with both engines at the tail.
My question is, which configuration adds more weight? With engines on wings, we must add pylons and strengthen the wing to resist bending, especially when on the ground. With engines at the tail, we must add weight to strengthen the T-tail. There might be some extra fuselage weight at the back for holding up the engines, but I'm not sure.
There may be other weight additions that I don't know about. Ultimately I want to know which config requires more weight.
Note: I'm not asking which is best. Best would require more than one factor. This question is only asking about one factor, weight.

The greater lateral distance from CG of most wing mounted engines over a tail mounted engine (consider engine out scenarios) means that a certain higher structure value will be required both for the greater thrust asymmetry as well as the greater control required, especially in the rudder.

I don't see how one could answer this. Engine placement is decided early on in the design cycle in response to criteria other than weight, and will have effects beyond the engine mounts. How would one then break out the weight that is solely due to the whereabouts of the engine?

@ReddHerring How would one then break out the weight that is solely due to the whereabouts of the engine? Seems pretty straightforward to me. All you have to do is ask the question, what stuff is there that wouldn't be needed in the opposite config? I don't see a problem. Pylons are a great example. Maybe there are many tiny components that are config-specific, and the list would be huge? If so i don't need an exhaustive list. I suspect all the major weight stuff is structural anyway, and maybe engine-related like generator size and placement.

Another way, theoretically, is to find two examples of twinjets, with the same passenger limit, but one has wing-engines and the other has tail-engines. But they would have to be designed in the same era or one of them would prolly be using lighter components. I have not been able to find any examples under this criteria.

@DrZ214 Well, it's easy then. Design a complete airframe based on one choice of engine location, then do the same design based on the other location. Who would do this?

Related: Why are modern regional jet designs shifting towards under wing engines?

Tail engines add more weight. Placing the engines in pods underneath the wing relieves the bending moment in flight, allowing for lighter wing root construction. Placing them at the tail requires local beefy frame construction.
Note that the upwards bending moment in flight is the limit design factor:

In flight all of the weight of the aeroplane times the ultimate load factor of 3.5, and this is the bending moment relieved by the weight of engines mounted at the wing.
On the ground, the undercarriage supports the weight of the aeroplane at load factor 1, with a smaller moment arm. Way smaller than the ultimate bending load.
At touchdown, the spring/dampers are dimensioned such that the bending moment at maximum sink rate is below the ultimate design bending moment.

Torenbeek 1 mentions the following on this subject:

Empty weight.
  The following factors have to be considered:

A wing structure weight saving is possible with wing-mounted engines due to the mass relief effect on the bending moment on the inner wing.
Engines placed too far outboard increase the landing impact loads and necessitate a large vertical tailplane.
Engines at the rear of the fuselage require local "beef-up" and lead to loss of useful space in the tail, resulting in added structure weight and a larger fuselage for the same payload.
Differences in weïght of the tail surfaces depend on various factors which do not permit a general conclusion.

Summing up, we may say without too much emphasis that the empty weight of a CaraveIle-type layout will typically be 2 to 4% more than that of a comparable design with the engines on the wing.

As mentioned in this answer, at the time of B737 design Boeing had two competing designs from different teams, and the wing mount won. For an Order Of Magnitude check we can compare the wing and fuselage weights as a percentage of OEW for the DC9 and the B737-100. The weight of the wing & fuselage groups as per the table in this answer, the OEW from Wikipedia:

So indeed if we look at the weight only, the underwing configuration wins. The tail configuration compensates with aerodynamic pluses, mainly from the clean wing.
Note that these considerations were of value at the time of DC9 construction, with the narrow engines of the time. A modern 200 pax plane with high bypass turbofans has no good reasons anymore to opt for the tail mount.

1 E. Torenbeek, Synthesis of Subsonic Airplane Design

Placing the engines in pods underneath the wing relieves the bending moment in flight... Yeah that part I get but I always thought, when the plane is on the ground, the bending moment is increased (as opposed to a wing with no engines on it), so you have to add structural wing-weight anyway. BTW can you tell us who Torenbeek is and put a link to that source doc so we can see exactly what it is too?

@DrZ214 https://www.springer.com/gp/book/9789024727247

It's not just bending moment. Underwing engines also act as anti-flutter weights (for this reason they are moved forward). 'Clean' wings often have dead-weight balances in their leading edge. On the other hand, underwing engines require longer and heavier landing gear for low-wing configurations.

Can a quad engine plane wing (A330 size) be substantially lighter in comparison to a twin with the same MTOW? Can we spread 4 lightweight engines across the span for better bending relieve? I'm thinking to create a separate question about it.

@ElectricPilot 4 engines delivering a certain amount of thrust would be heavier than 2 engines with the same thrust. The outboard engines require a larger vertical tail volume for engine fail. The outboard engines provide a larger downward wing bending moment upon landing impact. So - probably not.

@Koyovis I was looking at it from the same MTOW perspective. In this case a quad will have less installed thrust, and still climb better in one engine out case. Less thrust = less powerplant weight, less powerplant weight = less pylon weight etc.


text:
Why would seaplanes use a curved takeoff run?
I came across this interesting factoid in another SE Aviation answer (How flat does a runway need to be?):

seaplanes often use a curved takeoff run.

I'm trying to imagine why but cannot come up with a good reason.
Does anyone know why?

You landed on a curved lake?

@jamesqf every lake is curved

@andrewmh20: Obviously, I meant curved as in an L or C shape, not curved as in round, so that you don't have room for a straight takeoff run.  (And FWIW, I've flown over a lake in Northern California that's square.)

@jamesqf  I meant curved as in curved by the earth.

@andrewmh20: But that Euclidian curved surface is in fact flat in the spherical coordinate system of gravity, air pressure, and everything else relevant to taking off :-)

It can be used as a cross-wind takeoff technique. See my answer below.

Years ago now I gave a fair amount of instruction for seaplane ratings. I remember it quite fondly as seaplanes combine the fun of flying with aspects of the fun of boating. There were three conditions in which we used curved takeoff runs.
The most frequent curved takeoffs were on rivers, which tend to not run in a straight line, and even for those portions of a river that do, there may be rapids to stay clear of, so following the bend of a river or weaving between rapids was not unusual. In my case, the rivers were the McKenzie and Willamette, both in Oregon, USA. If you're ever in Eugene, Oregon, go down to Skinner's Butte Park on the Willamette River. On the upstream end of the park is the Ferry Street bridge. On the downstream end is the Washington-Jefferson bridge. In the 1970s we used to operate on the water between them, avoiding the rapids. We stored the airplane for a time on the Skinner's Butte Park shore line. That was legal and safe back then.
The next most frequent usage of curved takeoffs was to get off ponds that were not wide enough to allow a straight line takeoff. However, you can do it taking off in a curve along the perimeter of the pond. If you ever happen to fly over Daniels Field alongside Interstate 5 about 20 miles north of Eugene, Oregon, you'll see a lake/pond alongside the runway with a small island in the middle. There is no straight line you can draw on that pond that comfortably gives you room for takeoff, but you can circle the island and get into the air without difficulty.
The third circumstance we didn't have to use often was when we had to run across our own wake to be able to get up on the step on glassy water. For example, Waldo Lake in the Oregon Cascades is at 5,400 feet elevation. A J-3 cub with an 85 horse engine and with two people on board simply cannot get up on the step when the surface of the lake is a dead calm. However, if you get going in a curved path, you can get up on the step when you run across your own wake. Once you're on the step, you can then put the airplane up on one float and get enough airspeed to take off. Just as a matter of information, seaplane operation on Waldo Lake is now prohibited.

I'd love to see video of a seaplane taking off circling an island like you describe. Just sounds cool.

Is 'On the step' the same as the state of planing that boat hulls can achieve?

@Gusdor Yes, it is. There is much less drag then, and you can accelerate to a higher speed, which is needed in order to takeoff.

@Gusdor Essentially; basically a floatplane has to break the suction of the water - kind of like pulling your boot out of mud - and if the water isn't moving at all then it has to do that all by itself. If there are some waves, or a wake, or any kind of discontinuity in the water, then that gives the aircraft the momentary break it needs to overcome that initial suction. "The step" in this context refers to the step-up shape you see partway down the float (or hull); when you're "on the step" then you're using the part of the float forward of that step to hydroplane.

Excellent answer. I learned something new.  I had never realized that taking off from a glassy water surface was harder.  Thanks!

"weaving between rapids was not unusual" Really puts the other questions on the site about takeoff and landing in perspective.

a thoroughly deserved 100+ points!

@anaximander: Suction of the water is only possible with curved bottoms; for quite the same reasons as with curved wing surfaces. Therefore, planing boats have a straight bottom, and the step is placed where it is to put the center of lift into the right lengthwise location. The step does not break any suction per se.

One reason for a curved takeoff is the downwind arc method as described on page 4-14 of the FAA Seaplane, Skiplane, and Float/Ski Equipped Helicopter Operations Handbook (FAA-H-8083-23):

DOWNWIND ARC
The other crosswind takeoff technique results in a
  curved path across the water, starting somewhat into the
  wind and turning gradually downwind during the takeoff
  run. This reduces the actual crosswind component at the
  beginning of the takeoff, when the seaplane is most susceptible
  to weathervaning. As the aerodynamic controls
  become more effective, the pilot balances the side loads
  imposed by the wind with the skidding force of an intentional
  turn, as always, holding the upwind wing down
  with the ailerons. [Figure 4-17]
The pilot plans a curved path and follows this arc to
  produce sufficient centrifugal force so that the seaplane
  tends to lean outward against the wind force. During
  the run, the pilot can adjust the rate of turn by varying
  rudder pressure, thereby increasing or decreasing the
  centrifugal force to compensate for a changing wind
  force. In practice, it is quite simple to plan sufficient
  curvature of the takeoff path to cancel out strong
  crosswinds, even on very narrow rivers. Note that the tightest part of the downwind arc is when the seaplane is traveling at slower speeds.

Original at https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/seaplane_handbook/media/faa-h-8083-23-3.pdf#page=14


text:
Why would maximum fly-time (endurance) not coincide with max L/D operating point?
This question is motivated by another SE Aviation answer where @Peter Kampf writes the following two useful bits of information:

Aircraft like to fly near their optimum L/D ratio, where drag reaches
  its minimum.
Observation aircraft which want to optimize flying time will fly
  slower that what optimum L/D requires, especially if they use
  propellers.

Do aircraft use the majority of their fuel to overcome friction?
I'm trying to understand this more: Why would maximum fly-time not coincide with max L/D operating point?
Is this something to do with the jet engine fuel consumption characteristic? i.e. Does the minimum fuel-consumed-per-unit-time point not coincide with the optimum L/D point? But if your payload is fixed, so is your lift & hence your velocity choice that gives you that lift right?
i.e. For a given payload, lift is fixed. For a given lift, minimum drag leads to max fuel economy? And max economy means max dwell time? What gives.

For simplicity I assume we can ignore the climb / descent / acceleration etc. for now. And only consider the level flight segment at fixed speed.

Maximum dwell time or maximum endurance occurs when the power required is minimum. Hence, in this case, the maximum endurance speed is one where the power required is minimum, while in case of maximum range speed, the thrust required is minimum.
For maximum endurance, we must minimize the fuel consumed per unit time i.e. the fuel flow. For maximum range, we must minimize the fuel used per unit distance traveled.
In case of propeller aircraft, the fuel flow rate is proportional to the power produced. Hence, the maximum endurance occurs at a point where the power is minimum. For (turbo)jets, the minimum fuel flow occurs when the thrust is minimum. Hence the maximum endurance occurs when the L/D is maximum. For turbofans, it is somewhere in-between.

Consider a propeller aircraft in a steady, level flight. For determining the condition where the energy expenditure is minimum, we have,
$P = W (\frac{C_{D}}{C_{L}})V$
is minimum. For steady flight, we have,
$V = \sqrt{\frac{W}{\frac{1}{2} \rho S C_{L}}}$
This gives,
$P = \sqrt{\frac{W}{\frac{1}{2} \rho S}}(\frac{C_{D}}{C_{L}^{\frac{3}{2}}})$
Thus, for propeller aircraft, the minimum power and maximum endurance occurs when $\frac{C_{L}^{\frac{3}{2}}}{C_{D}}$, rather than $\frac{C_{L}}{C_{D}}$ is maximum. Due to this, the minimum power (maximum endurance) condition occurs at a speed which is 76% of the minimum drag (maximum range) condition.

Image from eaa1000.av.org
Also, see here and here

Thrust is a force that moves the aircraft. In steady, level flight, this is equal to the drag (if it is more/less, the aircraft will accelerate/decelerate). Power is the rate of doing work i.e. the energy consumed per unit time or rate of energy expenditure (by the a/c powerplant). This is why we are considering minimum power i.e. rate of energy expenditure for determining endurance.
Power is the product of force (thrust) and velocity. Think of it in this way- as speed increases, the drag decreases, reaches a minimum and then increases. However, as the power is product of drag (i.e. thrust) and velocity, it too follows the a similar path; however, the minimum is reached before the minimum drag. That speed gives the maximum endurance.
For jet engined aircraft, the speeds are different. In this case, the speed corresponding to minimum $\frac{C_{L}}{C_{D}}$ gives maximum endurance, while the speed corresponding to $\frac{C_{L}^{\frac{1}{2}}}{C_{D}}$ gives maximum range. Also, see here

Can you expand on the difference between Power and Thrust?  While it seems clear that max Range - where speed plays a role - happens at a faster speed than max Endurance (where speed does not), it isn't clear how overcoming MORE drag (i.e. by being left of L/D max) makes for better endurance. Is this relationship true for jets and props both? Thanks.

@RalphJ I've added some points about thrust and power. Hope this helps.

The polar point for maximum flight time only coincides with the minimum drag point when engine thrust does not change over speed. This is approximately true for pure turbojets and rockets. If thrust creation involves accelerating a big mass flow of air, the declining thrust with increasing speed for a given engine power shifts the optimum to lower speeds.
For a more general optimization goal we need to minimize not drag, but fuel flow. Since thrust varies with fuel flow for propeller and bypass engines (as it does for ramjets), this can be calculated when we model thrust $T$ over speed $v$ as $T \varpropto v^{n_v}$ with $n_v$ a negative number for propeller and turbofan engines, and positive for ramjets.
Starting from the equilibrium in steady flight
$$T_0\cdot v^{n_v} = c_D\cdot\frac{\rho}{2}\cdot v^2\cdot S$$
we can express the speed $v$ in terms of the lift coefficient $c_L$
$$T_0 = c_D\cdot\left(\frac{\rho}{2}\cdot S\right)^{\frac{n_v}{2}}\cdot\left(\frac{m\cdot g}{c_L}\right)^{1-\frac{n_v}{2}}$$
$T_0$ is the reference thrust at a specific speed and depends only on fuel flow. You can see it equally as the thrust setting, and we want to minimize this. Therefore, we approximate the drag coefficient with the quadratic polar ($c_D = c_{D0} + \frac{c_L^2}{\pi\cdot AR\cdot\epsilon}$), differentiate the right part of the equation with respect to $c_L$ and look for the lift coefficient at which it is zero:
$$0 = \frac{n_v-2}{2}\cdot c_{D0}\cdot c_L^{\frac{n_v-4}{2}} + \frac{n_v+2}{2\cdot\pi\cdot AR\cdot\epsilon}\cdot c_L^{\frac{n_v}{2}}$$
$$\Leftrightarrow c_L = \sqrt{\frac{2-n_v}{n_v+2}\cdot\pi\cdot AR\cdot\epsilon\cdot c_{D0}}$$
This by itself is not yet helpful, but if we look at the ratio of the drag components at specific values of $n_v$, the answer becomes clear:
$$c_{Di} = \frac{c_L^2}{\pi\cdot AR\cdot\epsilon} = \frac{2-n_v}{n_v+2}\cdot c_{D0}$$
Propeller aircraft ($n_v$ = -1): $c_{Di} = 3\cdot c_{D0} \Rightarrow$ 76% of the speed for lowest drag
Turbofan aircraft ($n_v$ = -0.5): $c_{Di} = \frac{5}{3}\cdot c_{D0} \Rightarrow$ 88% of the speed for lowest drag
Turbojet aircraft ($n_v$ = 0): $c_{Di} = c_{D0} \Rightarrow$ 100% of the speed for lowest drag
For optimum loiter speed, the induced drag of a propeller aircraft needs to be three times as big as the zero-lift drag. As induced drag drops with increasing speed, only for turbojets will the optimum polar point for maximum flight duration be equal to that at the lowest drag.
Nomenclature:
    $c_L \:\:\:$ lift coefficient
    $n_v \:\:\:$  thrust exponent, as in $T = T_0\cdot v^{n_v} $
    $\pi \:\:\:\:\:$  3.14159$\dots$
    $AR \:\:$  aspect ratio of the wing
    $\epsilon \:\:\:\:\:$  the wing's Oswald factor
    $c_{D0} \:$  zero-lift drag coefficient
    $c_{Di} \:\:$  induced drag coefficient

I'm still confused. Let's say we fly on a Cessna 172 at speed slower than its Vmd. We have now higher drag to overcome. Am I right in assuming, that by flying slower than Vmd, a propeller is actually a bit more efficient? As a consequence we can fly with lower RPM even if we are slower than our Vmd? Is this actually correct? Does lower RPM means lower fuel flow? Thanks and regards.

@ElectricPilot: Propeller thrust is inversely related to speed. As long as the drag increase by slowing down is less than the thrust increase, the difference will be greater, hence less power is required to stay aloft. It is not about propeller efficiency but how much thrust can be gained from a given amount of power. Power is thrust times speed.


text:
Aircraft with air bags for seats
Are there any (passenger) aircraft models with airbags for seats?
I had never heard of it but was watching a film where a Virgin Atlantic engineer mentions it for their business class and premium economy seats in the Dreamliner ( circa 2015)
Wondering if this is true and which other aircraft have this feature. Is this a premium class perk? Or is there something about the business seats that demands airbags.
PS. Is it easily visible if a aircraft seat has air bag protection?

https://en.wikipedia.org/wiki/Airplane_airbags

How much protection and in what situations would that really help during the crash itself and not hinder the post-crash evacuation?

I saw one on Cathay Pacific A330 for sure. Not sure about other airline or fleet. It is on economy class and easily visible. It is a large piece of equipment on seat belt and clearly label as such.


text:
Why offload bags for missing passengers when flights are allowed to carry unaccompanied non-PAX air-cargo in holds?
Several times, my flight has been delayed when passengers don't make it to the gate in time and then (after a long wait) the baggage-handlers have to go into the holds and manually find the bags to offload.
First, is this a legal requirement? Do all carriers follow this same practice?
If so, then why is this a security risk any greater than the same flight anyways carrying air-cargo in its hold? Flights do that, correct? Not all cargo space is filled with checked-in baggage?
Does anyone know the rationale behind this calculation? If the hold anyways contains unaccompanied cargo then why treat unaccompanied passenger baggage as a high risk?
Also, historically, is there a record of such offloaded bags (for missing passengers) actually turning out to be containing explosives?

When the situation is more nervous than usual, a variant of having the baggage handlers go into the holds and manually finding the bags to offload, is to unload all baggage onto the ramp and then have the passengers deplane and identify their baggage individually. Really time consuming. The one time it was done on a flight I was operating, it delayed us by around two hours as I remember.

In the 1990s at the carrier I flew for, air cargo on pax flights from JFK to Tel Aviv was accepted only from known and trusted sources, and when the security alert level was high, cargo was sometimes put in altitude chambers to check for barometric devices.

@Terry Interesting! Can you tell more? "More nervous than usual"? What are they trying to catch by doing this? A malicious loader?

The carrier's primary scheduled flying was JFK-TLV, and in some months we carried more pax on that route than El Al, the Israeli state airline. It was a Jewish-owned airline and, like El Al, as I understood it, we were considered to be at risk for terrorist activity. For example, at Amsterdam Schiphol, we were at times escorted to and from the runway by armored vehicles, one in front, one in back, and one at each wing tip just as El Al was.

@Terry: Which carrier were you flying for?

At the time, I was flying for Tower Air, a small (23 747s) JFK based carrier. I joined them in 1995, retired in 1999, and they went bankrupt in 2000 or so. Some of my experiences with them are detailed at https://terryliittschwager.com/emailjournal.php and https://terryliittschwager.com/talking-of-flying.php, although I have yet to finish that last link. Who knows whether I'll ever get that done.

I thought there was an offload rule, but recently I gave up my seat on an overbooked flight and by bag flew ahead of me on my original flight.  It was waiting for me when I got there on the later flight.  So it's not an absolute rule apparently

Partial answer:
The rule about offloading passenger bags is known as Positive Passenger/Bag Matching. It was introduced after the Pan Am 103 bombing in 1988 where a bomb was placed in a suitcase that was checked through to New York without belonging to any passenger on the plane.
By the practices of the time, all bags tagged with the right flight number would be loaded onto the plane, without pausing to cross-reference them to passenger manifests. The investigation never found out for certain how the suitcase with the bomb had entered the baggage system (an inside job at Luqa airport was suspected but not proved), but the outcome was to require matching of all the bags to actual boarded passengers before departure.
So this is not necessarily a case where explosives were placed in the checked bag of a passenger who should have boarded but didn't -- but it is close enough to see that the risk is not pure fantasy.
The rule applies to all international passenger flights, but not to US domestic flights; the US authorities apparently have better trust in their ability to screen checked bags for explosives before loading them onto a plane.
I would assume that air cargo also undergoes similar screening and that is why it is not subject to PPBM rules -- even outside the US there is better time to screen cargo thoroughly than there is for bags that are only checked an hour or two before departure.

This Travel.SE question and its answers have more information about PPBM.

Thank you! Do you mean that in US domestic flights they don't necessarily offload bags for missing Passengers?

@curious_cat: That's what I understand, but I have it only second-hand through the Travel question I linked.

That's true. I've certainly missed a US domestic connection only to find that my bag arrived before me. The US did practice PPBM for a few years, but I don't know when they stopped.

Info rejected as an edit, but worth keeping: Positive passenger bag match was already being conducted by certain air carriers before the bombing of Pan Am 103. When Air India Flight 182 was bombed in 1985, the International Civil Aviation Organization (ICAO) mandated PPBM for all flights. Therefore, PPBM was already mandated when Pan Am 103 exploded. The investigation determined the process was not conducted by Pan Am. (Jeffrey C. Price, lead author, Practical Aviation Security: Predicting and Preventing Future Threats).

There are a number of questions in here, but there is one common misconception.
Unaccompanied baggage: Positive passenger bag matching (PPBM, offloading of unaccompanied baggage) is not required by US, EU, or ICAO standard.
ICAO Annex 17

4.5.3 Each Contracting State shall ensure that commercial air transport operators do not transport the baggage of persons
  who are not on board the aircraft unless that baggage is identified as unaccompanied and subjected to appropriate screening.

If the unaccompanied baggage is screened via approved advanced detection systems, or hand-searched, it may fly without the passenger. Of course, an airline can merely offload the baggage if they do not have the proper screening.
The appropriate screening standards are set out in EU EC 2320/2002 5.2.2 and 49 USC 44901.
The US set the eventual requirement for all baggage to be screened with advanced systems by default. Additionally, the EU allows normally-screened baggage to fly without the passenger if the passenger is separated from the bag due to reasons outside their control.
Air cargo: Post 9/11, the US implemented the "known shipper" program which requires all cargo to be from TSA pre-approved shippers with approved security programs. Airlines cannot accept cargo from the public.
One change that resulted from this are that passenger aircraft cannot carry parcel mail (only lightweight letter mail). Additionally, cargo carried on passenger aircraft is now screened the same as passenger baggage.

Thanks! What's some scenarios where "passenger misses the flight due to reasons outside their control" but bag makes it?

@curious_cat It would be the opposite most likely, a passenger makes the flight but the bag doesn't. They can then send the bag by the next flight. Perhaps one scenario is the passenger is held up by immigrations or security but their bag isn't. I changed the wording slightly.

@curious_cat a few years ago I was stuck at AMS due to snow and my original connection was cancelled - at 11.15pm, KLM put us on the 11.30pm flight, and we ran to the gate to get it but they had closed the gate.  As a backup, we were also put on the 6am flight the next morning so we got that instead.  Our bag was waiting on the conveyor at our destination about 2 minutes after we got off the plane (small regional airport), so it had made the earlier flight with no problems...  Everyone else on the plane had to wait for their bags to come.

Baggage may be transported unaccompanied IF it has been appropriately screened, but checked baggage should not be carried if the checking passenger is not on the same flight (by their own actions!). These rules were brought in in the days before suicide bombers because terrorists (and extortionists) would check a bag with a bomb and then choose not to get on the flight.


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What is this 'runway number' automatic readout system on a flight deck?
I was watching this flight-deck video (starts at 11:13) of a Turkish Airlines Boeing take off from Istanbul and just as they accelerate on the take off roll there seems to be an automatic voice in the background reading out the runway name, "Runway 35 Right".
Is this a standard safety feature? Never heard it before. I've only heard the ATC give the clearance manually.
Is it an aircraft feature or an airport system or a combination? Is it meant to prevent incidents where pilots line up on a parallel taxiway / runway?


text:
Has there ever been a seaplane with retractable pontoons?
I would like to know if there's ever been a seaplane in which all of its pontoons or sponsoons are retractable after takeoff.
This could be a seaplane with 1 or 2 main pontoons that somehow retract. Or more likely, this could be a flying boat (bouyant hull) whose under-wing pontoons retract into the wing. I will also count a flying boat with only sponsoons that can retract somehow.

There have been a couple of examples- the Blackburn B-20, for example, had a retractable pontoon below the hull (and retractable wing floats), though it was an one-off experimental aircraft with a single flight to its credit.

Blackburn B20, image from dinger.byethost5.com
As a more widespread example, the Consolidated PBY Catalina's wing floats were retracted electrically after takeoff.

Catalina's retractable wing float; image from navy.memorieshop.com
Retractable floats have been used in other aircraft too- like the Grumman G21A Goose and the SL-12C amphibian. Some designs like the Kawanishi E15K featured retractable floats, which were later removed due to issues.
A little more digging reveals that the German Ursinus Gotha WD 10 Fighter Seaplane had retractable floats quite early- during WWI!. However, I doubt it counts as the aircraft never went past taxiing stage.

Ursinus WD 10 fighter seaplane, image from 1000aircraftphotos.com


text:
In a fighter jet, what is the average weight of the ejection system?
In the old days, pilots had only a parachute pack. (In the really old days they might not even have that.) They had to open the canopy manually and jump out.
Today, fighter jets have zero-zero ejection seats. They can save the pilot even at zero airspeed and zero altitude. They do this with solid rockets to eject the pilot upwards.
I would like to know what is the average weight of this system for a single-pilot fighter jet? That way we can compare it's weight to the total weight of the plane and see what fraction it is.
Really the weight would include only the rockets and explosive bolts on the canopy, and I guess the ejection handle too. It would not include the weight of the seat or the pilot, as those have to be there even without an ejection system.

@ymb1 Are you saying the seats need softer cushions for it? Or special cushions? If so then yes definitely include it. I still say the seat structure itself should not be included unless it's reinforced to withstand the g-force. I don't know if old-school ww2 seats could withstand it as is.

@ymb1 Right, the seat and pilot eject as one, so it has to be designed for that. I'm just saying exclude the seat itself because a seat would exist in any fighter jet even without an ejection system. I'm trying to understand how much weight is added to include an ejection system. However, 100 kg is about 1.4% of a 7 ton fighter jet, so even including the seat I think it is very small addition.

I can't address the collection of fighters, but the U-2 seat is 248 pounds, without pilot, prior to firing.
Source: Co-worker who flew U-2s.


text:
How are modern jets modified to takeoff/land on a dirt runway?
I've been reading a lot about the Soviet and Russian military aircraft, and it seems a lot of them are built to work on dirt runways, or modified to do so.
How is this accomplished? Is it a simple thing to modify aircraft in this way? Is it just a matter of strengthening the undercarriage and upgrading the tires for more stress? Or is something harder involved, like maybe strengthening the fuselage frame?
NOTE: let's exclude worries about foreign objects going into the engines. Right now I'm only interested in the difference between paved and dirt runways as far as landing gear and strength are concerned.

Related: How heavy is too heavy to land on a grass strip? and also Can jet airplanes operate on 'rough' airfields?

FOD damage/ingestion is actually the larger concern of jet aircraft designed to land on unimproved strips. Actually touching down and rolling around on packed dirt is fairly academic.

@KeithS I had a feeling this was true. Nevertheless the undercarriage part is the only part I'm interested in, and it's a completely different type of modification from any engine modifications afaik. I don't really mind if an answer talks about FOD safety too, I just wanted to be clear what I'm interested in.

Just to roll around on "unimproved" airfields (grass/dirt/gravel strips), the biggest thing planes need is simply a decent set of tires:

This MiG-29UN trainer shows off the fairly beefy main tires put on these planes specifically to allow them to more easily roll over minor imperfections in the runway/taxiway surface, including dirt/gravel/grass strips (though here it's landing on a traditional asphalt/concrete surface). Compare these to the relatively small gear wheels and low-aspect rubber on the similarly-sized F-16:

(source: f-16.net)
Now, your question oversimplifies things, because "foreign object debris" or FOD is in fact a primary danger of unpaved airstrips for jets, and this danger has to be accounted for in the design. Here's the main reason the MiG-29 can take off from dirt:

MiG-29s and similarly-minded Su-27s, and their variants, have special doors to close off the lower intakes normally used in flight, instead drawing in air through the gill-like ports over the top of the wing strakes. This prevents the engine from ingesting dust, rocks, birds, rodents, etc on takeoff and landing.
This engine protection system, while effective, adds mechanical complexity and weight which U.S. designers and military brass have generally thought unnecessary given that requirements rarely include the ability to operate from unimproved strips. Most U.S. fighters in the jet age, therefore, have had to operate from airbases with some form of paved runway surface (and the U.S. military actually has higher standards for runway surface quality than most international airports). The U.S. arsenal does, however, have several aircraft that are specifically designed to operate from rudimentary airstrips at forward bases. These designs avoid FOD damage primarily by keeping the engines up as high as the design will allow. For instance, the A-10's two engines are on pods angled up on the fuselage between the wings and tail for a variety of reasons including FOD avoidance:

Most of the other designs in the Western arsenal that can do this are high-wing designs with wing-mounted engines, which keep them out of the dust produced by takeoff and landing in a similar fashion. Here are a C-130 and C-17 landing at an Afghanistan forward base:

The Harrier is a notable exception to this general strategy by Western designers of just keeping the engines out of any debris cloud. While it's probably not done routinely, a Harrier can set down just about anywhere, and as this picture shows, it ingests no small amount of dust in the process:

Yay! Pictures! :)

Interesting about the tires, some are thick and some are thin. Is tire pressure also important?

Tire pressure is important, you basically want a slightly lower pressure so the tire will flatten out and increase its footprint over uneven ground. But, it still has to roll and to support the aircraft, so the tire itself is made bigger to allow for the "squish" without bottoming out on touchdown.

Then can I ask what would happen if the F15-E tried to land on dirt? Would the tires blow, the landing gear collapse, and it would require major repairs? Or is it possible to survive with just a few kinks?

If an F-15E tried to land on dirt, what would happen depends on the exact surface. Smooth, hard-packed dry dirt wouldn't be very different from a dusty asphalt runway, which F-15s have been using in the Middle East for 20 years at least. Any larger gravel, stones or dirt clods creating irregularities in or just under the surface will start causing problems, up to and including the gear snapping off on touchdown as the wheel is too small or too rigid to roll over the bump at high ground speeds.

There is, however, a difference between using a dirt field and operating from one. In an emergency, a pilot's trained to land anywhere he can. In such a situation the plane would then take off again with minimal fuel load and no ordnance to get back to its actual operating base. To actually fly a mission, an F-15 can be loaded down with more than its own weight in fuel and ordnance. The runway surface then has to be smooth enough to minimize shock forces to the gear at max takeoff weight (or the gear must be strong enough to handle the shock forces expected from a dirt strip at MTOW).

The two main issues faced by aircraft landing on rough runways are:

Ingestion of FOD in the engines, and
Damage to the landing gear.

In case of the rough landing, one of the best ways is to distribute weight using larger and higher number of tyres. For example, compare the landing gear of C-17,

"A U.S. Air Force Airman inspects the landing gear of a C-17 Globemaster III aircraft, July 1, 2014, on the flight line at Joint Base Charleston, S.C. The maintainers performed checks and maintenance around 140701-F-EV310-009" by A1C Clayton Cupit - http://www.defenseimagery.mil/imageRetrieve.action?guid=18df79c5d609dbaa0679ab32d9e6aea6e76200c6&t=2. Licensed under Public Domain via Wikimedia Commons.
with that of Boeing 777, which has about the same maximum takeoff weight (MTOW).

Source:
Aircraft like Su-27 use a FOD deflector in their wheels to reduce the damage due to FOD.

Source: maybach300c.blogspot.com
One way to keep the engines clear of FOD is to have them mounted high on the wing, where the FOD ingestion is reduced.

Source: www.defense.gov
Or, the engines can be mounted high on the fuselage.

Source: theaviationist.com
In case this is not possible in the case of a high-performance combat aircraft, the inlet design has to be modified. The Russians used this extensively as their operational doctrine demanded it. For example, the MiG-29 main engine inlets could be closed for operation from rough runways.

Source: www.reddit.com
When the main inlets are closed, the air for engine is obtained through inlets on the top of the wings, which are shielded from the debris below.

While the  C17's tires are significantly wider than the 777's (estimated a a 2:1 height:width ratio vs 3:1), both photos show a groups of 6 tires and assuming the persons shown with them are equally tall they're about the same height. (Both are roughly 2.5x the ankle to knee distance.)

There are things called "gravel kits" that allow standard commercial aircraft to land on rough(er) surfaces.
https://en.wikipedia.org/wiki/Gravel_kit

Would you please describe the modifications to the Air North plane?

Additional descriptions indeed! The "fender" on the NLG seems fairly obvious (and appears to form part of the NLG door once retracted), however that probe under the engine is a bit confusing.

The vortex dissipators are fascinating. If you’ve ever seen a vortex from an intake reach down the ground, you’ll know what does.  It gets in the way. https://m.youtube.com/watch?v=p5l3fD2WIQc


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What makes a smokeless jet engine?
I was recently reading about the MiG-29 and its many upgrades, one of which (MiG-29M) was to smokeless engines.
What makes a smokeless jet engine? Is it just the fuel, or something about the way the engine works itself?
I've observed smokey engines were more common in the past and I'm wondering why that is too. Were they just easier to build for some reason?

Just as a guess: more efficient combustion. Smoke is basically not-fully-burned bits, so the more of the fuel you burn, the less smoke you have. (How you get that, I don't know, which is why I'm not posting an answer :) ).

Compressor stall will make a jet engine smokeless, but this condition is to be avoided during flight.

@HowardMiller I'd think flame-out would be more likely to produce a smokeless engine than compressor stall (though the latter could lead to the former, but tends to produce significant amounts of smoke in the process.)

@reirab, I have to admit I haven't seen many flameouts, but yeah, compressor stalls tend to be pretty dramatic. When I was at Cam Ranh Bay, the usual aircraft launch plan tended to be to get going as fast as you can on the runway, rotate and climb away as fast as possible. For supersonic fighters, this meant launching on afterburners, and I never saw much smoke. C-130s, which have turbine engines, pretty much the same. C-124s inject a water alcohol mix into the engines on takeoff and the tended to have more of a smoke tail.

Smoke results from soot in the combustion gasses, so the carbon part of the hydrocarbon fuel was incompletely burned.
The literature (sorry, paywalled, but you can read the first page) reports that in the first decades of jet engine design much combustor development was trial and error. Specifically, to arrive at a smokeless combustor, the JT9D team had to perform 465 modifications and 140 full engine tests. Mainly, these tricks help:

Slow flow speeds by careful diffusor design at the compressor exit
Swirling of the flow at the combustor entry to improve fuel-air mixing
Measured inflow of air into the combustor, such that the fuel injection area has an essentially stoichiometric mixture for best flame stability. Then more air is added in a secondary and a third dilution region. This dilution helps to lower the gas temperature so the gas stream is palatable to the turbine.
Longer combustors, so that 99% combustion can be achieved at the end of the secondary region.

Selecting the right fuel helps, too. If you would burn methanol or ethanol, a smokeless combustor is easier to achieve, but the energy density of the fuel suffers. Lighter fuels with shorter hydrocarbon chains help, but are more expensive to produce and reduce energy density, so kerosene was preferred even if its combustion produces soot.

Generally, smoke comes from incomplete burned fuel.
Normally, a jet engine burns a very lean mixture (due to turbine temperature constraints). Nevertheless, getting all the fuel to burn is a considerable challenge, primarily because the air/mixture flows faster in the engine than the flame front. So in order to provide stable burn, many tricks come into play: the combustion chamber expands after the compressor; there are special vortex generators that force the mixture to backflow and circulate; the air may be fed in stages along the chamber, etc.
This complicated gas dynamics is difficult to optimise, especially in the 70s when MiG-29's engines (RD-33) were designed. What usually happens is that there are pockets of over-rich mixture in the combustion chamber, and the extra fuel decomposes due to high temperature before it has an opportunity to burn.
From what I know, RD-33MK got a redesigned combustion chamber (amongst other things), but what exactly the changes are is hard to say. I can speculate that the source of the original problem was that RD-33 had an early example of the annular combustion chamber, which was not well studied then. But the problem was bad enough to make MiG-29 more easily detectable in visual fight.

Isn't lean burn hotter than rich one?

@Agent_L Not if you have enough excess air that it dilutes the exhaust gasses 5-10:1. I believe it's a stoichiometric (ideal) mixture that burns hottest, because there is no excess fuel or air to heat.

@Agent_L, carburated, spark-ignited piston engines, like still used in many GA planes, are always operated rich-of-peak, because they tend to run rough lean of peak due to unevenness in the fuel distribution. So there leaner means hotter, because it is closer to stoichiometric. But turbines (and compression-ignition engines) run lean with a lot of excess air and leaner means colder there.


text:
Has there ever been a fully automated takeoff, cruise, and landing of a large aircraft similar to commercial airliners?
Has there ever been a fully automated takeoff, cruise, and landing of a large aircraft similar to commercial airliners?
In fact, has there ever been a fully automatic flight of any pure aircraft whatsoever? This is gonna sound crazy, but the only thing I know that comes close is the Soviet Space Shuttle Buran. It launched unmanned and returned successfully under computer control, including its glide and landing.
So I'm interested in automatic aircraft similar to large commercial airlines. BTW, remote control toy planes wouldn't count because that is not automated (a person is manually controlling it).

NYTimes article about state of the art

I think it's safe to assume you're talking about normal VFR conditions with no traffic, no ATC clearances needed, and limited to no mechanical failures.  See http://aviation.stackexchange.com/questions/1802/why-do-we-still-use-pilots-to-fly-airplanes for more possible issues that human pilots deal with manually.

The American military space vehicle the X-37 does the same thing as the Buran did. Some F16s and other aircraft have been outfitted with controls that allows them to serve as target drones but it's unclear how automated the flight paths are.

Didn't the mythbusters episode about talking someone through landing say that autopilot could land the plane?

The book The Glass Cage describes a fully autopilot-controlled military test flight in 1947. The plane was a C-54 Skymaster with seven men aboard. According to this source, the pilot aligned the plane on the runway for takeoff, but the takeoff, course control, and landing were performed by autopilot with no human engagement. See also this report from the Chicago Tribune at the time.

Great find, thank you! And welcome to the site!

I'd note that it doesn't specify whether the actual landing was performed by the aircraft: only that it tracked the glideslope. It may be that the pilot finished the landing - that wouldn't surprise me, as it's quite a tricky thing for an aircraft to do without a radar altimeter

@JonStory, "At dawn the following day, the C-54 reached the English coast. Still under the control of the autopilot, it began its descent, lowered its landing gear, lined itself up with an airstrip at a Royal air Force base in Oxfordshire, and executed a perfect landing. Captain Wells then lifted his hands from his lap and parked the plane." I'd say the plane did all the work...

Whoops, stopped reading too early!

Fully autonomous flights have been demonstrated- but not not in 'large commercial' aircraft.
The Dassault Aviation AVE-D 'Petit_Duc' drone has demonstrated fully autonomous flight:

The flight, watched by representatives of France's Délégation Générale pour l'Armement (DGA) armaments procurement agency, comprised a completely automated sequence: roll from parking spot, runway alignment, takeoff, in-flight maneuvers, landing, braking and rolling back to the parking apron.

Helicopters also have demonstrated this ability. A Boeing-modified MD530F helicopter has demonstrated fully autonomous flight:

... this time, the helicopter did it entirely on its own — with no humans involved. It was the first fully autonomous flight of a full-sized chopper, ever.

Unmanned Little bird, image from wired.com
Note: I've considered only those where it is explicitly mentioned that 'full' flight was autonomous. Not the capability alone.

Interesting, but who or what is in that photo lying on his back while a robo heli flies around? lol.

@ymb1 Why did you delete your answer? It was a good one. You could've just added a note.

I don't see how. The popsci article you linked said that takeoff and landing were done manually. If it's just automated cruise then I don't see how that's much different from autopilot.

@DrZ214 Its a dummy. They are testing the helicopter for SAR operations, I guess.

if you want you could add the DA42, last year we did a full automated flight (FlySmart/Lufo project)

@DrZ214 - I'm going to say that's Buster. :)

Not as big as a commercial airliner (but bigger than many people realise), at the time of its introduction into the USAF, Global Hawk flew from Edwards Air Base in California to Australia without any human control (including takeoff and landing).
Following incidents like 9/11 and GermanWings, it would be sensible to introduce a feature into all passenger airliners now to automatically land at the nearest airport - and once the feature had been invoked, it could not be revoked. Better yet, largely remove most of the pilot's work.

Adding a feature like that as a response to infrequent catastrophes sounds like it will cause more trouble that it will fix.

It's an excellent idea, in the sense that automation is far more consistent than any human could ever hope to be.  But then there's the automation paradox, which basically means that it will get the humans out of practice for situations that the installed automation honestly can't handle.  Now you're riding an airplane in trouble without an autopilot, and the human pilot is effectively inexperienced.  Is the occasional situation like that worth thousands of prevented accidents due to pilot error or hijacking?  You decide.

The problem with being unable to revoke the feature is that you can end up killing all your passengers. First, the nearest airport may not be suitable for landing for any of a large variety of reasons. Even if it is, it could well be the case that landing at the nearest airport is a considerably worse idea than landing at the second or third nearest airport. Beyond that, conditions (such as crosswind) can change during the landing such that conditions are no longer suitable for landing, even though they were earlier.

The first paragraph is interesting, but would be improved if a source could be provided. The second paragraph is kind of off-topic commentary. Also, that suggestion opens up a huge can of worms... like how the feature is activated in the first place. Most answers to that cause more problems than they solve.

There is no current aircraft capable of fully automated flight in all stages of flight as equipped and operated according to the POH. The closest you can get is from climb out to landing. Currently you could fly the takeoff and activate the auto pilot once established on climb out (probably some time after the gear came up or around then). Let the FMS fly the plane, then setup the auto-land to take it home. But you still need to get it off the ground.
There are various ways to guide a plane in flight via automated systems. Most modern airliners contain an FMS unit capable of using a variety of navigation methods and control of steering. Smaller planes that don't utilize an FMS may have GPSS (GPS Steering). GPS Steering is becoming more common on smaller planes from what I have seen.
Auto-land capabilities have been around for some time, one of the earliest planes to have it was the Concorde. There are many airframes that now contain auto-land capabilities but they are not utilized all the time.
It should be noted that all of these things need to be programed by the pilot before the flight. Any deviations by ATC while in route would also need to be programed.


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What is this "Micro T-tail" on the F-102?
I just noticed this unusual thing on the F-102 Delta Dagger. It looks like a micro T-tail. What is it actually, and what is its purpose?

It’s an antenna.

Related: What is the purpose of the winglets on the Socata TB-31 Omega vertical stabilizer?

The T tail is the VHF Antenna Localizer of the AN/ARN-31 Instrument Landing System.

source: Convair F-102 Delta Dagger Pilot's Flight Operating Manual


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Do airline dispatchers pre-block seats for weight and balance on large wide body aircraft?
I was reading a Oman Air flight review and one of the bits intrigued me:

I had earlier used ExpertFlyer to monitor both how busy the flight
  would be and the seat map. I selected a window seat when I checked in
  and noticed there were plenty of empty seats around me. On the day of
  the flight, I noticed the two seats next to me were showing as
  blocked, which I assumed was part of a weight and balance exercise.

This review is for a wide body aircraft (Boeing 787 Dreamliner) and I am just wondering if Dispatchers ever block seats for weight and balance like the review writer refers to.
I mean I have often seen large blocks of seats not available for selection sometimes in the online interface but I always assumed this was for commercial reasons and not technical. In smaller flights I have seen passengers being moved around for weight and balance but is this also done in the large wide body aircraft?
Does anyone know?

Related, if not dupe: Are passengers really not allowed to change seats before take-off due to weight and balance?

Yes sometimes, because with most aircraft you can't fill all of the seats and all of the fuel tanks and stay under max gross weight, meaning the airplane can't achieve its potential maximum range with a full cabin and on long range city pairs some sort of passenger/fuel horse trading may have to go on.
So on flights where the airplane needs more range that it has with a full cabin, it may be forced to limit seats filled to accommodate the extra fuel, and it will distribute the empty seats throughout the loading zones to prevent load concentrations for balance reasons.
You will also see seats blocked off to accommodate dead heading crew, but if you see a significant number blocked off in each cabin loading zone, and the seats are still empty when you board and its a long flight, it was probably due to fuel requirements.